Sulphur dioxide is a harmful pollutant that must be removed from stack emission before it can be safely discharged into the environment.
Commercially feasible techniques have been developed whereby sulphur dioxide is separated through processes that draw the sulphur dioxide into a liquid which is retained when the cleansed gas is expelled. In such processes sulphur dioxide is either absorbed (as sulphur dioxide) into water or some other liquid or, alternatively, otherwise reacted with chemicals in a liquid solution. Scrubber towers house structures that promote this type of cleansing process by maximizing the emission-gas to liquid interface across which the sulphur dioxide is drawn into the liquid.
Inside the scrubber towers are multiple levels of substantially horizontal shallow trays across which the absorptive or reactant liquid flows. The stack emission-gas bubbles through the depth of liquid on each tray as a pressure differential drives the gas through small apertures and flows across it to the downcomer by which means it transfers to the next lower tray. The liquid flows across this second tray to another downcomer, repeating the process across a series of descending trays while a flow of emission-gas, entering at the base of the tower, ascends through the same series of trays.
Ordinary applications require towers of considerble size, generally much too large to allow a tower to be factory constructed and then transported to location. Rather, typical current construction techniques use concrete tower walls slip-formed at location as the primary supporting structure. A round steel tower of conventional field erected construction is sometimes substituted for the concrete tower. In either case, a perimeter support structure is constructed at location and field construction continues as bracing, supports, trays and other internal structures are assembled and installed into the perimeter support structure. However, on site construction is subject to several disadvantages when compared to factory construction. Economic factors favor factory construction because labor and other costs of field construction tend to run higher and because field construction incurs additional expenses in bringing complete construction teams and equipment to the site. In addition, the conditions of field construction are less controlled and prove more difficult to supervise. These latter alternatives of field construction not only make field construction more expensive, but makes quality control more difficult and subjects the entire duration of the construction process to a dependence upon favorable weather. Nevertheless, on site construction remains the industry practice, even though more of the construction might be done at the factory.
One principal constraint currently limiting factory construction is the common practice described above of using poured-in-place concrete tower construction. Though factory construction is dependent upon a major change in the basic design, exterior support remains a primary consideration as all internal structures need corrosion resistant properties at a cost which restrains the use of major internal supporting structures. Furthermore, internal support members waste tower space, since they do not function directly in the scrubbing process.
Another constraint limiting factory construction is the size of the tower which prohibits the final assembly in the factory as well as the economical transport of the assembled tower to location.
Prior workers building other chemical process equipment such as baghouses, have attempted to deal with some similar problems by so reducing the size of the equipment that, at best, completely assembled functional units can be made at the factory, be ganged into batteries at location, and perhaps be installed with common duct work. See U.S. Pat. No. 3,680,285.
That approach is "unitized" in a sense in that it joins modules together, but each module is an independent functional unit, that is to say that no joining of constiutent subunits is necessary to produce a functional unit. This approach, as applied to scrubber towers, is lacking because construction on a reduced scale producing functional scrubber tower units of transportable size greatly decreases the proportion of the functioning members, here trays, to other structures.